The invention relates to a drive arrangement for a motor-vehicle top having an outward-moving sliding cover that is actuated by an electric motor, a transmission, and at least one threaded cable engaging a pinion of the transmission. The circuit of the electric motor comprises a manually-actuated switching means provided with a contact arm which, in the closed position of the sliding cover, cooperates with a cam of a trip gear to stop the electric-motor current supply. With an indexing gear driven by the pinion shaft, the toothed trip gear forms a step-by-step indexing transmission whose ratio is determined so that the trip gear rotates by a maximum angle of 360.degree. while the sliding cover moves from its fully retracted position to the closed position and an upwardly fully extended position.
In a known drive arrangement of this type (German Gebrauchsmuster [Utility Model] No. 7,629,034), each revolution of the cam gear corresponds to a rotation of the indexing gear by a predetermined angle. Practice has shown that the known arrangement requires that the transmission elements be manufactured within narrow tolerance limits because, due to the high transmission ratio of the step-by-step indexing transmission, the length of the indexing step available to actuate the limit switch is very short. In addition, the response is determined by the mobility or inertia of the sliding cover and the ambient temperaure influencing the electric-motor rpm. Especially when the sliding cover is readily movable and the ambient temperature is relatively high, the limit switch may not be actuated in the closed position of the sliding cover, i.e. not respond in the closed position. Then the sliding cover undesirably moves beyond the closed position.
Basically, the electric motor can be slowed near the closed position by short circuit, polarity reversal, or actuation of magnetic brake. But such electromechanical structures are complicated, and therefore unsatisfactory.
Thus, a principal object of the invention is to provide a drive arrangement of the above-cited type which enables reliable actuation of the limit switch, therefore fundamentally eliminating switching errors, and therefore also creating a particularly simple, compact, and low-cost structure.
This object is achieved in accordance with a preferred embodiment of the invention by providing the step-by-step indexing transmission with transmission elements designed to extend the length of the indexing step of the trip gear near the closed position of the sliding cover, and/or to reduce the trip-gear angular velocity during said indexing step.
With this structure of the invention, the limit switch for interrupting the electric-motor current supply in the closed position of the sliding cover is given sufficient time for reliable operation, independently of the degree of mobility of the sliding cover and the ambient temperature. Satisfactory operation is assured even in the case of indexing gears of wider tolerance limits. Subsequent adjustment of the limit switch is unnecessary.
An especially simple structure appears in the embodiment of utilizing the trip gear of FIGS. 7, 8 in conjunction with the indexing gear of FIGS. 11 and 12. The indexing gear can be mounted directly on the pinion shaft. The load applied to the limit-switch contact arm is especially light when the cam cooperating therewith is in the form of a negative cam provided on the periphery of the trip gear.
In another development of the invention, the trip gear may also carry a cam surface to actuate an rpm-control switch connected to the electric motor and change the motor speed from a higher value used to effect the sliding motion of the sliding cover to a lower value during the portion of the sliding motion which immediately precedes the closed position and the outward-extension motion. Therefore, the time available to actuate the limit switch is increased, and also a sensitive control of the sliding-cover extension motion is provided.
In addition to producing a speed change during closing, a cam surface can be shaped so that the change over to a lower rpm can occur also in the last portion of the sliding motion, before the fully open position is reached. Thus, the sliding cover moves smoothly and surely to the end position.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in accordance with the present invention, and wherein: